Single-crystalline, cubic ZrO2- and HfO2 nanobelts have been fabricated via a direct thermal-decomposition process under atmospheric pressure. The formation of cubic nanobelts can be controlled by varying the dopant concentration of nanobelt precursors, heating temperatures, and heating rates. The lanthanide-doped nanobelts show a dual capability for both down- and up-conversion emissions (see picture) in the visible range.